1. Field of the Invention
This invention relates to vehicle suspension systems, and more particularly to a method and apparatus for selectively modifying a suspension parameter in response to changes in the relative orientation between telescopically movable components of a hydraulic damping device.
2. Description of Related Art
To provide means for selectively adjusting a suspension parameter or for controlling variable leveling requirements, information regarding the relative motion of the suspension and/or the vehicle body is required. Such information is commonly supplied by sensors to a central electronic controller for measuring and calculating suspension and body displacement, velocity and acceleration. This information is used by the electronic controller to selectively control the suspension characteristics of the vehicle to provide the desired response.
Damping devices ("dampers") are used in conjunction with automotive suspension systems to absorb unwanted vibration (impacts, loads, etc.) which occurs during driving. To absorb this unwanted vibration, dampers are generally connected between the body and the suspension of the automobile. A piston is located within the damper and is connected to the body of the automobile through a piston rod. Because the piston valving and orifices act to restrict the flow of damping fluid within the working chamber of the damper when the damper is compressed, the damper is able to produce a damping force which counteracts the motion of the wheel and/or body which would otherwise remain undamped. The greater the degree to which the flow of damping fluid within the working chamber is restricted by the piston, the greater the damping forces which are generated by the damper.
In selecting the amount of damping that a damper is to provide, three vehicle performance characteristics are often considered: ride comfort, vehicle handling and road holding ability. Ride comfort is often a function of the spring constant of the main suspension springs of the vehicle, as well as the spring constant of the seat, tires, and the damper. Vehicle handling is related, among other things, to variation in the vehicle's attitude (i.e., roll, pitch and yaw). For optimum vehicle handling and, consequently, superior body and wheel control, relatively large damping forces are required to avoid excessively rapid variation in the vehicle's attitude during cornering, acceleration, and deceleration. Road holding ability is generally a function of the amount of variation in the normal load between the tires and the ground. To optimize road holding ability, larger damping forces are required when driving on irregular surfaces to minimize the normal load variations so as to prevent complete loss of contact between the wheels and the ground.
To optimize ride comfort, vehicle handling, and road holding ability, it is generally desirable to have the damping forces generated by the damper be responsive to the frequency of the input from the road or from the body. When the input frequency is approximately equal to a natural frequency of the body (e.g., approximately between 1-2 Hz), it is generally desirable to have the damper provide relatively large damping forces (relative to critical damping) to avoid excessively rapid variation of the vehicle's attitude during cornering, acceleration and deceleration. When the input frequency is between 2-10 Hz (mostly from the road), it is generally desirable to have the damper provide low damping levels so as to produce a smooth ride and allow the wheels to follow changes in road elevation. When the input frequency from the road is approximately equal to the natural frequency of the automobile suspension (i.e., approximately 10-15 Hz), it is desirable on one hand to have relatively low damping forces to provide a smooth ride, and on the other hand provide high damping forces so as to minimize variation in tire normal load and prevent complete loss of contact between the wheels and the ground.
Selective control of a desired suspension parameter often requires information regarding the movement of the piston within the pressure cylinder of the damper. This information not only identifies whether the damper is in compression or extension, but also can provide information concerning the magnitude and frequency of suspension motion.
Several methods are known for obtaining information regarding the movement of the piston within the pressure cylinder. PCT Application No. PCT/US87/00615 uses a pressure sensor as well as an accelerometer to determine whether the damper is in compression or extension, as well as to obtain information regarding the road surface. U.K. patent application No. GB 2 177 475A and West German patent No. G 87 02 817.4 disclose suspension damping devices incorporating ultrasonic "sonar" wave systems for determining positional displacement information. The positional displacement information is obtained by determining the time from transmission of an ultrasonic wave to when its reflected "echo" wave is received. Both references use a single transducer acting to emit and receive the pulsed ultrasonic waves. Use of a single transducer necessitates incorporation of costly ultrasonic wave modulation and calibration circuitry to ensure coherent wave detection. Additionally, the transducers in both references are mounted such that the piston acts to reflect the ultrasonic waves.